The invention relates to a method and mechanism for the thermal treatment of pulverulent or finely grained material, and particularly for the calcining of cement in several steps whereby the raw material is first preheated in a precalcining step and finally the calcining is completed in a furnace. More particularly, the invention relates to improvements in the precalcining step in cooperation with the calcining step so that controlled heating and controlled oxidation takes place in precalcining as the material is admixed with partially oxidized fuel and oxygen.
In the production of cement, a conventional process involves calcining and sintering which take place consecutively in a rotary kiln in two separate portions so that the rotary kiln solely performs a sintering function, and another part connected in series operates on added calcining material particles in an airstream. This is shown, for example, in German Pat. No. 446,029. In accordance with this principle, the precalcining device is a cyclone heat exchanger, and permits the rotary kiln to be made smaller in size. In the rotary kiln, because of the dynamic stress that occurs with rotation, a more expensive structure is involved than with a heat exchanger that does not rotate but is subjected only to static load. Thus, if it is possible to reduce the size of the rotary kiln, this brings about a savings in cost of construction and also a savings in space required, and this can be accomplished by a more effective improved precalcination structure and operation.
For such precalcination, a heat supply is provided in a separate special combustion zone. In this combustion zone, it is important that a predetermined maximum temperature must not be exceeded because excessive temperatures will result in the danger of caking of the material interfering with the normal operation of the device. Attempts have been made to provide separate special combustion chambers which have been relatively expensive from a structural standpoint, and these are positioned between the preheater and the rotary kiln functioning for sintering wherein a special complex intermixture of raw material and fuel is attempted to prevent peaks in temperature, and such chambers insuring this function have been found to be expensive in construction and not wholly satisfactory.
It has also been proposed to carry out precalcination in the gas conduit leading to the actual preheater as is shown in German Laid Open Specifications Nos. 2,324,519 and 2,324,565. In order to prevent the possible overheating and resultant disadvantages, in this type of device the fuel is mixed with raw material to be calcined before the fuel is distributed into the heated discharge gas stream emerging from the rotary kiln. The mixture takes place at a location where the concentration of raw material is so high that no combustion occurs. Such a concentration of raw material at predetermined points of the gas conduit results in a concentration of raw material which is relatively low at other points. A supply of fuel at these low concentration points leads to high peaks of temperature with resultant disadvantages.
With the present invention, the aforementioned difficulties have been overcome in a simplified and reliable way. A reliable ignition of the combustible fuel as well as a uniform combustion at relatively low temperatures has been insured. The fuel is ignited and partially oxidized in precombustion step and combined with the hot oxygen containing gas stream from the furnace, and this mixture is admixed with a substantial part of the raw material to be treated which is further oxidized in a precalcination zone. In accordance with the invention, partial oxidation first takes places with the fuel supply, and the fuel is ignited reliably and uniformly. In the first combined completely ignited and partially oxidized condition, it is admixed with the majority of the raw material to be treated. The partially oxidized fuel is there further burned in intimate mixture with the material to carry out the precalcination. In accordance with the method, a completely uniform distribution of the combustible material and the raw material take place, and delays in ignition or combustion as have occurred with methods heretofore available wherein too great a density of material present, are reliably prevented. Local overheatings are also prevented.
It is accordingly an object of the present invention to provide an improved mechanism and method for the calcination of cement wherein a smaller main calcination burner and furnace may be provided reducing the cost by diminishing the size of moving parts.
A further object of the invention is to provide an improved method and structure for precalcination of cement wherein a more uniform and a more effective precalcination takes place to improve the capacity of precalcination apparatus.
A still further object of the invention is to provide an improved precalcination arrangement wherein a more uniform mixing occurs between heated gases and oxygen and raw material, and wherein a particularly more uniform heating of the material occurs without the possibility of localized areas of high temperature occurring.
A still further object of the invention is to provide an improved arrangement wherein a partial oxidation of the fuel takes place and oxygen and partially oxidized fuel is supplied to an admixing area for uniform distribution of the fuel and oxygen through the material and the completion of combustion of the fuel and oxidation of the material to be precalcined.
In one embodiment of the invention, partial oxidation of the fuel takes place in the hot oxygen containing gas stream that is received from the calcination furnace, and thereafter the mixture of partially oxidized fuel and the gas is combined with the raw material. In this arrangement, parts of the normal oxygen containing streams of gas are used for the partial oxidation of the fuel. After the ignition and preoxidation, the mixture of partially oxidized fuel and gas is combined thoroughly with the raw material, and upon the residual burning up of the fuel, the precalcination is completed with the optimization of use of total fuel energy.
In a further embodiment, additional oxygen is supplied to the stream of gas after partial oxidation at a controlled rate. This addition of oxygen to the oxygen in the gas coming from the rotary kiln can be adjusted as desired to adapt to varying operation of the rotary kiln and varying rates of material treatment. This further enables reduction in the size of the main rotary kiln and permits accurate control of the combustion of the gases by control of the location and amount of addition of additional oxygen. In this arrangement, a flame guidance structure is provided which has a separate preoxidation zone, and an admixing area for the calcination zone wherein the calcination zone is optimally positioned relative to the position of the ignition chamber, combustion chamber and burner as well as the location and direction of additional fuel supply and oxygen supply nozzles.
For carrying out of the method of the invention, the main sintering furnace or kiln are provided with a standpipe or vertical conduit provided for receiving at its lower end the furnace exhaust gases. Fuel is added in a preoxidation zone in the conduit wherein the fuel is partially oxidized with the oxygen from the exhaust gases. Material feed is supplied above the fuel feed with the distance between the fuel feed and material feed being critical so that no raw material reaches the area of fuel introduction and the fuel flow path is limited so that only partial oxidation takes place before admixing occurs. In accordance with the vertical relationship and the selected distances between fuel introduction and material feed, a completely uniform flame can develop so that an accurately adjustable temperature profile can be obtained which is not disturbed throughout the combustible material. With this arrangement even with a high material density and wherein nonuniformity and material concentration occurs, there is no resultant ignition retardation or combustion retardation which would adversely affect the quality of the precalcined product. The afterburning effect which has occurred nonuniformly in installations heretofore available is reliably prevented. This arrangement also prevents material from being held up in stationary spirals and remaining in a location too long wherein overheating can occur.
In a further arrangement of the invention, a deflecting device is located between the precombustion zone and the admixing area where the raw material is added. This deflecting device prevents larger particles of material from dropping downwardly and entering the fuel ignition and preoxidation area. The deflected materials are carried upwardly by the stream of gas and do not reach the oxidation chamber. This prevents particles of material from disturbing the preoxidation and flame distribution.
In another embodiment of the invention, a combustion chamber is provided for a partial oxidation of fuel with the combustion opening into the gas conduit leading from the rotary kiln. In this arrangement, a partial preoxidation occurs in the separate chamber, and adjustment of fuel and oxygen supply can be made in accordance with the requirements of flame expansion and speed of ignition. Such a chamber makes it possible for completely uniform preoxidation, and the development of a temperature profile favorable to the thermal process required in the precalcination zone. This separate combustion chamber may be arranged concentrically about the gas conduit and enables a uniform distribution peripherally into the gas conduit.
A preferred distance between the fuel feed through the preoxidation zone to the point of material supply to the admixing area is one-fourth of the theoretical flame length. Thus, the material introduction in the admixing area takes place in the most favorable hot part of the combustion. The distances between one-fourth of the theoretical flame length and not greater than the flame length are preferably much closer to one-fourth of the flame length so that the combustion of the flame is not so cooled by the stream of cooler raw material flowing into the oxidizing flame so that unequal or overheating will not occur.